The present invention relates to a high-intensity discharge lamp with a built-in starter.
In this energy-saving era, as high-intensity discharge lamps for outdoor use such as in streets, public squares, avenues, or for indoor use such as in factories, sports arenas, and shops, metal halide lamps and high-intensity sodium lamps are used because they are more efficient and provide better color rendering in a comparison with conventionally-used high-intensity mercury lamps.
Since these metal halide lamps and high-intensity sodium lamps are applied in general to conventional facilities that have been used for conventional high-intensity mercury lamps, the lighting requires a simple copper-iron type reactance ballast based on the power supply frequency. Therefore, for the purpose of lighting by means of the copper-iron type reactance ballasts, these lamps contain starting circuits that are not provided for conventional high-intensity mercury lamps.
Various types of starting circuits have been used depending on the lamps, which are classified in general into the following two basic types.
An example of a first basic type is shown as a starting circuit 57 in FIG. 6. The circuit has a basic structure of a series circuit comprising a switching element 58 for interrupting current, a resistor 59 for restricting current, and a thermally-actuated switch 60 composed of a bimetal for disconnecting the starting circuit. This series circuit is connected in parallel to an arc tube 63 comprising at both the terminals a pair of main electrodes 61 and 62.
The aforementioned starting circuit 57 and the arc tube 63 are arranged inside a glass outer tube 65 that is under vacuum or filled with a gas so as to compose a lamp 64. The switching element 58 can be, for example, a glow starter for a metal halide lamp, a nonlinear ceramic capacitor or a thermally-actuated switch of a bimetal for a high-intensity sodium lamp. The thermally-actuated switch functions also for disconnecting the starting circuit.
The bimetal thermally-actuated switch 60 for disconnecting the starting circuit, as shown in FIG. 7, comprises a lead 66 as a fixed contact, an insulating glass 67, a supporter 68 as a L-shaped fixed electrode member, a contact rod 69 as a movable contact, and a bimetal plate 70 as a movable electrode member. The lead 66 is connected at one terminal to the electrode 61 and connected at the other terminal to the insulating glass 67. The supporter 68 is connected at one terminal to the insulating glass 67 and connected at the other terminal to the resistor 59. The bimetal plate 70 is provided with the contact rod 69 at the front end portion while the back end portion is attached to the supporter 68. The contact rod 69 contacts with and/or separates from the lead 66 due to slow turn-over operation of the bimetal plate 70 caused by heat. A portion that the fixed contact of the thermally-actuated switch 60 contacts with the movable contact, i.e., a contact between the lead 66 and the contact rod 69, is positioned to be exposed to the interior of the outer tube 65.
The starting circuit 57 operates in the following manner. When the switching element 58 repeats on-off operation by application of a supply voltage 22, a high voltage pulse ranging from 1 kV to 4 kV is induced at a reactance ballast 21 due to interruption of current at every time of on-off operation, thereby causing the arc tube 63 to start discharging. Subsequently, the on-off operation of the switching element 58 stops just after the discharging starts. About two to three minutes after the start of the discharging, the thermally-actuated switch 60 shifts slowly from a closed state to an open state by the heat from the arc tube 63, and thus, the starting circuit 57 is disconnected from a lighting circuit. Subsequently, the thermally-actuated switch 60 maintains its open state during the steady lighting state of the lamp.
Regarding the starting circuit 57 in FIG. 6, a typical metal halide lamp using a glow starter for the switching element 58 is provided with a resistor 59 arranged in the vicinity of the thermally-actuated switch 60, so that the thermally-actuated switch 60 shifts from a closed state to an open state due to heat from the resistor 59 so as to stop the switching operation of the glow starter in case of a starting failure of the arc tube.
A second basic type is exemplified as a starting circuit 71 in FIG. 8. Such a circuit is used particularly for a metal halide lamp 76 using a quartz arc tube 75 comprising an auxiliary electrode 74 as well as a pair of main electrodes 72, 73. This starting circuit 71 comprises a series circuit including a resistor 77 for restricting current and a thermally-actuated switch 78 of a bimetal for disconnecting the starting circuit. The starting circuit 71 is connected at one terminal to the main electrode 72 and to the auxiliary electrode 74 at the other terminal. Also for this thermally-actuated switch 78, the contact is positioned to be exposed to the interior of the outer tube 79.
The starting circuit 71 operates as follows. When a supply voltage 22 is applied, auxiliary discharge occurs first between the main electrode 73 and the auxiliary electrode 74. Next, due to the action of initial electrons sufficiently supplied from the auxiliary discharge, main discharge starts between the main electrodes 72 and 73. About two minutes after the start of the main discharge, the thermally-actuated switch 78 shifts slowly from a closed state to an open state by heat from the arc tube 75, and the starting circuit 71 is disconnected from the lighting circuit. Subsequently, the thermally-actuated switch 78 maintains its open state during the steady lighting state of the lamp.
Some kinds of metal halide lamps use the above-mentioned two basic types of starting units together.
However, it has been known through a long-time use on the market that in the metal halide lamps and the high-intensity sodium lamps containing such conventional starting circuits, especially the above-mentioned two basic types of starting circuits, problems will be caused in connection with thermally-actuated switches for a basic components of such lamps, which are used for disconnecting starting circuits.
As mentioned above, the thermally-actuated switch 60 or 78 comprising a bimetal used for such a conventional starting circuit 57 or 71 has exposed contacts, since such a structure is cost-effective and problems like oxidation are not caused as the contacts are housed in an outer tube.
In a lamp using such a thermally-actuated switch 60 or 78, especially when the arc tube 63 or 75 fails to start or it ceases its lighting at the end of life etc. due to rise in the lamp voltage, arc discharge can occur, even though the possibility is low, at a contact of the thermally-actuated switch 60 or 78 in an OFF state, i.e., an open state. This is caused by a high voltage pulse induced at the reactance ballast 21 due to current interruption. Here, the problem is that the initial electrons supplied from the arc discharge at the contact can induce sustained occurrence of further arc discharge between a pair of leads that hold the arc tube. Because of the sustained arc discharge, excessive lamp short-circuit current may run continuously in the reactance ballast 21. Moreover, terminals of the outer tube 65 or 79 facing a lamp base may be damaged although the possibility is low as well.
While an outer tube of a typical high-intensity sodium lamp is in a vacuum state as mentioned above, materials such as sodium as a luminescent material and a xenon gas for a starting aid may leak from the interior of the arc tube at the end of the lamp life. Experimental results show that this causes the above-mentioned sustained arc discharge. Arc discharge can occur at the contact of the thermally-actuated switch 78 of the second type starting circuit especially when the arc tube breaks off. The reason is considered as follows. Since a conducting state between the main electrodes and the adjacent auxiliary electrode is maintained just after the arc tube breaks off, a high voltage pulse induced due to the interruption of the lamp current will be applied directly to the disconnected contact of the thermally-actuated switch.
Occurrence of excessive lamp short-circuit current, damage in an outer tube caused by arc discharge sustained inside the outer tube or the like, should be avoided from an aspect of safety for a high-intensity discharge lamp that comprises a movable electrode member of a bimetal and contains a starting circuit using a slow-action type thermally-actuated switch that turns over slowly when the temperature reaches a predetermined level. Secure solutions of such problems require the prevention of occurrence of sustained arc discharge induced by arc discharge at a contact of a thermally-actuated switch.
The present invention provides a high-intensity discharge lamp with high safety, suppressing inducement of sustained arc discharge inside an outer tube caused by arc discharge at a contact of a thermally-actuated switch when an arc tube fails to start or breaks off at the end of the lamp""s life or the like.
A high-intensity discharge lamp according to the present invention contains a starter, i.e., the lamp comprises an arc tube having a pair of main electrodes, a starting circuit having a thermally-actuated switch for disconnecting the starting circuit, and an outer tube containing the arc tube and the starting circuit, where the lamp is lighted up by means of a reactance ballast. The thermally-actuated switch comprises an envelope bulb that covers contacts of the thermally-actuated switch.
The structure can prevent the occurrence of sustained arc discharge in the outer tube, which is induced by arc discharge between contacts of the thermally-actuated switch when the arc tube fails to start or breaks off at the end of the lamp""s life.
It is preferable that the thermally-actuated switch is a snap-action type thermally-actuated switch.
It is also preferable for the thermally-actuated switch that a spacing between contacts in an open state is at least 0.3 mm.
In the above-mentioned structure, the arc tube has an auxiliary electrode, while the starting circuit comprises a series circuit of a glow starter, a first resistor, and the thermally-actuated switch, and the series circuit is connected in parallel to the arc tube, and a second resistor having one terminal connected to a node between the thermally-actuated switch and the first resistor and the other terminal connected to the auxiliary electrode, where the first resistor and the thermally-actuated switch are arranged adjacent to each other.
An alternative structure comprises a start-aiding conductor that is arranged to be supplied with voltage via a capacitor along with an axial direction of the arc tube, and the starting circuit comprises a series circuit of nonlinear ceramic capacitor having a switching function to interrupt current, a tungsten filament resistor for restricting current, and the thermally-actuated switch, and the series circuit is connected in parallel to the arc tube, and a heating resistor that is connected in parallel to the tungsten filament resistor and to the nonlinear ceramic capacitor and arranged in the vicinity of the nonlinear ceramic capacitor.
Alternatively, the arc tube can comprise an auxiliary electrode, and the starting circuit can comprise a series circuit of a resistor and the thermally-actuated switch, and a terminal of the thermally-actuated switch that is not connected to the resistor is connected to a main electrode while a terminal of the resistor that is not connected to the thermally-actuated switch is connected to the auxiliary electrode.